Much progress has been made in the last thirty years in developing optical switches or modulators, but current devices are not very satisfactory for many applications. The majority of active fiberoptic devices used in present day systems, for example, fiberoptic intensity attenuators, are based on electromechanical operation. In one type, fibers are positioned end to end and mechanically moved in or out of line. In another type, mirrors are rotated to direct beams into or away from a receiving fiber. This can be accomplished mechanically or with piezoelectric or electrostatic drivers. Mechanical devices intrinsically lack speed and long term reliability. Solid-state light controlling devices (without moving parts) are needed for fiber communication systems. A key problem for these developing fiberoptic components is realizing speed and reliability, as well as the essential fiberoptic systems requirement of low insertion loss and polarization insensitivity. For devices used between regular fibers, low insertion loss and polarization insensitivity operation is the basic performance requirement.
Others have proposed an optical switch/attenuator using a liquid crystal cell as the modulation element situated between an input and an output birefringent element, each fed by optical fibers. When the liquid crystal cell is turned on, light emerging from the output birefringent element is deflected and not focused by the subsequent collimator onto the corresponding optical fiber. Although it has the desirable features of low insertion loss, and low required operating voltage, being liquid crystal-based, the long term reliability of organic materials and the relatively low switching speed are not suitable for many applications.
Others have also proposed a fast (less than one microsecond) optical switch using an electro-optic crystal in which birefringence can be induced by application of an electric field. Operation is based on rotating the plane of polarization of light with respect to the orientation of a subsequent passive polarizer that blocks or transmits light depending on the angle. The basic arrangement works efficiently with incoming light polarized with a particular orientation. Randomly polarized light suffers a loss. This is overcome by using additional elements that split incoming light into two orthogonal polarizations, passively rotates one to match the other, and combines the two into a single beam fed to the basic modulator. However, the suggested electro-optic crystals, require voltages of a kV or more for operation.
Still others have described a modulator having a tapered plate, a Faraday rotator or electro-optic crystal, and a second tapered plate. The Faraday rotator is controlled by varying the current in an external coil which varies a magnetic field. The suggested electro-optic crystals require high drive voltages of kilovolts. Electrode design also effects polarization dependence and modulation efficiency.
Accordingly, the main objects of the invention are to provide an electrically controllable solid state optical modulator, attenuator, or switch that is insensitive to the polarization of the incoming light, has low insertion loss and, has a fast (one hundred nanoseconds or less) response time. Another object of the present invention is to provide a system for compensating the solid state devices against environmental changes, for example, temperature. Additional objects are to provide a device using rugged oxide materials and using easy assembly and alignment processes.
These objectives and other features and advantages are realized in two basic modes. In the transmission mode, arbitrarily polarized light beam enters from one side (the input surface) and exits the other side (the output surface). In one embodiment, the modulator comprises, between the input and output, a polarization separator, e.g., a birefringent plate with an oriented c-axis, followed by an electro-optic phase retarder with electrodes to generate an internal electric field when a voltage is applied, followed by a polarization recombiner. The separator breaks the light beam into two polarization rays, an ordinary one having a polarization direction (angular orientation with respect to the separator c-axis) perpendicular to the c-axis and an extraordinary one with a polarization direction parallel to the c-axis. In addition, the extraordinary ray is deflected in a plane containing the c-axis while the ordinary ray travels straight through. These two paths define a separation plane. The recombiner doesn""t effect ordinary rays either, but causes extraordinary rays to be deflected an equal amount but opposite the separator deflection back to be recombined with undeflected ordinary rays at the output. The modulator is normally-on. The phase retarder has an electric field that extends across the optical path at an angle, preferably at about 45xc2x0 to the separation plane which is also at 45xc2x0 to both the extraordinary and ordinary polarization directions. When a voltage is applied to the phase retarder, portions of the extraordinary ray become ordinary and are not deflected to the output. In addition, portions of the ordinary ray become extraordinary and, instead of traveling through the recombiner to the output are deflected away from it. With sufficient voltage, the two rays are completely interchanged so that none of their components reach the output.
A normally-off modulator can be obtained simply by orienting the deflection of the recombiner to be in the same direction as the separator. If the output is placed equidistant between the undeflected ordinary ray and the twice deflected extraordinary ray, none will normally reach the output. However, if a voltage is applied to the phase retarder, portions of the ordinary ray will be deflected once and portions of the extraordinary ray will be not be deflected and both will reach the output. With sufficient voltage, all light will reach the output. Addition of a 90xc2x0 polarization direction rotator, i.e., a polarization direction interchanger, to the normally-off modulator produces a normally-on modulator with low polarization mode dispersion. Addition of two 45xc2x0 polarization direction rotators allows the fields in the phase retarder to be at 90xc2x0 to the separation plane which produces a modulator with the minimum spacing between phase retarder electrodes thereby reducing the control voltage.
In a reflection mode, the simplest version comprises a separator covering an input area and a transversely displaced recombiner covering an output area, both followed by an electro-optic phase retarder, in turn followed by a reflector which directs the rays which have traveled through the separator and retarder back through the retarder for a second pass and then through the recombiner to the output. Having the input and output on the same side is considered useful in certain applications. A further advantage is that having two passes through the phase retarder means that each pass adds to the phase so that less voltage is required for full modulation. In full modulation, linear polarized extraordinary and ordinary rays with polarization directions at 45xc2x0 to the electric field become circularly polarized on one pass and rotated by 90xc2x0, i.e., interchanged, after two passes.
As in the transmission mode, the deflection of the recombiner can be arranged to provide normally-on or normally-off modulation. The control voltage can be reduced by adding a 45xc2x0 polarization direction rotator, e.g., a half-wave plate with a c-axis at 22.5xc2x0+Nxc3x9745xc2x0 (N an integer), between the separator/recombiner and the phase retarder so that the electric field can be at 90xc2x0 to the separation plane. Insertion of a circular polarizer, e.g., a quarter-wave plate with a c-axis at 22.5xc2x0+Nxc3x9745xc2x0 (N an integer) will convert any configuration from normally-on to normally-off and vice versa.
The described modulator/attenuator can be built advantageously to control power levels in, for example, fiberoptic communication systems. In these applications the I/O ports are made of optical fibers and can be assembled in transmission or in reflection mode. In particular, the transmission and reflection mode assemblies can be made advantageously using Graded Index lenses (GRIN lenses). For a reflective system, one side of the lens can be made reflective by e.g. coating the lens surface or attaching a mirror. The other side of the lens receives the input light and emits the output beam. The two input/output fibers must be symmetrically located on both sides of the optical axis of the GRIN lens. For ease of alignment the fibers can be mounted on a single fiber block and aligned simultaneously to the optimal position. This type of alignment eliminates a full degree of freedom and makes the fiber attachment considerably more expedient.
The phase retarder can be made from a special class of ferroelectric complex oxides in the form of polycrystalline ceramics which are optically isotropic, but become anisotropic along the direction of an applied electric field. In other words, the field makes them birefringent with a higher index of refraction along the field than perpendicular to it. An example is lead lanthanum zirconate titanate (PLZT). The electric fields for full modulation are higher than for liquid crystal phase retarders, but the response time is much faster.
According to another aspect of the invention, a system to provide transmission as a function of control voltage without hysteresis, comprises a compensator for an electro-optic device that can be characterized as having an optical input port, an optical output port, and an electrical control port connected to an electro-optic phase retarder that controls optical transmission through the device.
In one such embodiment, there is provided a stable light source having a selected amplitude directed toward a second reference attenuator comprising a first polarizer, a second electro-optic phase-retarder that can be constructed from the same material as the first phase retarder in the main optical attenuator, and a second polarizer aligned with respect to the first polarizer to function as an analyzer. In addition, a light sensor detects the output from the analyzer and applies a voltage to the inverting input of an amplifier. The amplifier output is connected to the second phase retarder forming an electro-optic feedback loop and also to the first phase retarder in the electro-optic device. When a voltage is connected to the non-inverting input of the amplifier, it produces an optical amplitude at the light sensor with a desired attenuation of the light from the stable light source that is environmentally stable. If the electro-optic device is calibrated against the reference attenuator, the matched phase retarders produce an environmentally stable electro-optic device. In particular, if the electro-optic device is an attenuator, its transmission is controlled by the input voltage to the reference. Further, if the phase retarder material is a ferroelectric with hysteresis, those effects will be mitigated. This embodiment can be used to control an electro-mechanical (piezoelectric) actuator if the actuator is substituted for the electro-optic device provided the movement of the actuator is calibrated against attenuation in the reference attenuator.
In another embodiment of the present invention, additional apparatus comprises a first beam splitter for deflecting a portion of the beam directed to the input port, a first light sensor for measuring the amplitude of the deflected portion the input beam, a controllable electrical attenuator for attenuating the output of the first light sensor, a second beam splitter for deflecting a portion of the beam directed to the output port, a second light sensor for measuring the amplitude of the deflected portion of the output beam, and an amplifier for amplifying the difference between the output of the electrical attenuator and the second light sensor. The amplifier output is connected to the control port of the optical attenuator forming a feedback loop so that the output of the optical attenuator is controlled by the setting of the electrical attenuator.
The same approach may be used to control absolute transmission by providing a beam splitter on the output port and a stable light sensor and amplifying the difference with respect to an input signal.
According to another embodiment of the present invention, for use with ferroelectric devices, especially having hysteresis, there is provided a capacitor having a selected value connected to one side of the electrical control port and the inverting input of an amplifier with its output connected to the other side of the electrical control port so that a feedback loop is formed. When a voltage is applied to the non-inverting input of the amplifier, the effect of the capacitor and feedback loop is to produce a controllable charge on the electro-optic material that remains relatively constant with environmental changes.
The above and other features and advantages of the invention, including various novel details of construction and combination of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular system and methods embodying the invention are shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention.